| Aluminum alloys are widely used in various products of underwater vehicles due to their low density,easy forming and high specific strength.The aluminum single-layer ring-ribbed cylindrical shell is mostly used as the main pressure hull in torpedo,mine,UUV(Unmanned underwater vehicle),and some other underwater vehicles.However,with increase the working depth,the aluminum single-layer ring-ribbed cylindrical shell exhibits poor external pressure stability,uneven stress distribution,and stress concentration.In order to meet those challenges,the double-layer cylindrical shell has been developed.Compared with the single-layer ring-stiffened cylindrical shell,the double-layer cylindrical shell structure exhibits a number of advantages including good compressive stability,uniform stress distribution,conforming to the concept of lightweight design and equal strength design,better matching of stress and material strength.Moreover,it can also effectively improve the utilization rate of materials.However,due to the lack of systematic research on the key technologies of the structure,cracking failure occurred in the girth weld connecting the inner and outer shell,resulting in batch accidents.In order to solve the weld cracking problem of a certain type of UUV double-layer cylindrical shell,this paper focused on the key technology of design and manufacture of double-layer cylindrical pressure-resistant structure.Based on the classical theory of elasticity,the stress calculation formulas of typical parts of double-layer cylindrical shells were deduced,and a new structural scheme was proposed.Emphasis was laid on the redesign of the weld structure,and the TIG welding method was adopted to weld the circumferential weld.The fatigue failure mechanism of the circumferential weld was discussed.The quantitative detection system of ultrasonic phased array defects was also constructed.Moreover,the research on the evaluation method of forming quality of the double-layer cylindrical shell under low cycle fatigue was carried out,which can guide the design,manufacture,inspection,and test of a certain type of UUV double-layer compressive cylindrical shell.This research has contribution to the engineering practice,which has improved product design and manufacturing quality,enriched new ideas and methods of UUV pressure structure design and manufacture,and promoted the application of double-layer cylindrical shell in underwater vehicle.The main conclusions were as follows:1)The mechanical model of double-layer cylindrical shells under typical conditions of uniform hydrostatic pressure was established and the stress solution method of typical parts was obtained.Based on the stress calculation method of typical parts of submarine pressure tank,the longitudinal pressure coefficient was deduced again,which can be used to calculate the stress of typical parts of double-layer cylindrical shell with good calculation accuracy.The calculated values of stress in typical parts of the theoretical stress calculation formula were in agreement with those calculated by the finite element method for establishing that the model and method were reasonable and effective.2)A new structure design scheme was proposed and tested.Aiming at the problem of weld cracking of double-layer cylindrical shell in the original scheme,combined with the theoretical stress calculation formula,the structure of double-layer cylindrical shell was redesigned,and a new structural design scheme was put forward.The new scheme reduced the stress peak in typical parts of the shell and improves the uniformity of stress distribution,reduced the requirement of material strength,and improved the rationality of welding structure and welding technology.The superiority of the new scheme was verified and the accuracy of the theoretical stress calculation formula for typical parts of the double-layer cylindrical shell was verified by the test.3)The welding parameters of 7A05 / 5A06 dissimilar aluminum alloy were determined by orthogonal test.TIG / MIG welding method was used to weld dissimilar aluminum alloy girth weld of double-layer cylindrical shell.The microstructure,mechanical properties and fatigue behavior of dissimilar aluminum alloy welded joint of double-layer cylindrical shell were studied.The results showed that the coefficient of TIG / MIG welding joint were 78.9%and 76.3% respectively(compared with 5A06-O base metal),the yield strength were157.3MPa and 151.6MPa,the elongation is 11.4% and 9.2%,and the average particle size of the welded joint is about 52 μm and 65 μm.The micro-hardness of FZ zone was the smallest and the tensile fracture occurred in FZ zone,and the heat-affected zone on 7A05 side was obvious.Compared with the MIG welded joint,the TIG welded joint has higher strength,superior ductility and toughness,and more typical fracture morphology of dimple fracture.The main reasons for the decrease of the strength of the welded joint are the evaporation of magnesium,the decrease of the second strengthening phase and the coarseness of the grain.4)The fatigue properties of 5A06-O,7A05-T6 aluminum alloys and 5A06-O/7A05-T6 dissimilar aluminum alloy annular welded joints with double-layer cylindrical shells in dry atmosphere were studied experimentally,and the mechanism of the influence of welding defects and loads on the fatigue properties of welded joints was revealed.The results showed that welding defects have adverse effects on the fatigue properties of 5A06-O/7A05-T6 welded joints,and the interaction between defects and fatigue loads were strengthened with the decrease of stress level,and the fatigue life was shortened more significantly.The fatigue cracks of 5A06 and 7A05 base metals usually initiated at the defects such as surface biphasic particles and inclusions,while stress concentration was more likely to occur at welding defects of 5A06 and 7A05 welded joints.The fatigue cracks were initiated in advance,which further shortened the fatigue life.5)The forming quality evaluation method of double-layer cylindrical shell under low cycle fatigue was studied.A quantitative detection system of ultrasonic phased array weld defects was constructed based on the weld structure of double-layer cylindrical shell in the new scheme,and a performance evaluation method of double-layer cylindrical shell circumferential weld based on ultrasonic phased array weld defect detection and fracture/fatigue failure evaluation method was formed and acceptance criteria are proposed accordingly.The rationality of the acceptance criteria is verified by the simulation test of double-layer shell girth weld area and the low cycle fatigue simulation test of a full-size double-layer cylindrical shell in actual use.Based on this criterion,the quality of 40 circumferential welds of 20 shells,which have been accepted and delivered,was reliable and stable.This paper systematically studied the key technologies of design and manufacture of double-layer cylindrical shell,such as mechanics theory,manufacturing method,nondestructive testing method of weld seam,and quality evaluation of weld seam,which combined theoretical analysis,mathematical modeling simulation and experimental verification.This research is useful for the concrete engineering practice of design,production,inspection and acceptance of double-layer cylindrical shell.Moreover,it has produced great economic value and greatly promoted the application of double-layer pressure-resistant structure in underwater vehicles,especially in the field of UUV. |
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